Vacuum tubes including rotating anodes bombarded by energetic electrons are well developed and extensively used, particularly as X-ray tubes wherein the anode includes a rotating X-ray emitting track, usually made of tungsten, bombarded by electrons from a cathode. X-rays emitted from the track are transmitted through a window in a tube envelope. The anode is rotated so at any instant only a small portion thereof is bombarded by the electrons. Even though the energetic electrons are distributed over a relatively large surface area, anodes of high power tubes of this type frequently are heated sufficiently to become incandescent in response to the bombardment.
One previous technique advanced to assist in cooling such an anode is the placement of a relatively high thermal conductivity liquid metal film in the thermal pathway between the rotating anode and a stationary heat removing structure. The liquid metal is usually gallium or a gallium alloy; gallium is used because it has a sufficiently low vapor pressure to be compatible with the low pressures within the vacuum tube envelope. Nearly all of the gallium remains in liquid form from 30.degree. C. to several hundred degrees centigrade. Gallium melts at a temperature of 29.78.degree. C. Certain gallium alloys, specifically binary and ternary eutectics, are frequently used because they melt at lower temperatures, near the melting temperature of water ice.
German Patent Publication DE 3644719 C1 discloses an X-ray tube including a rotating anode track irradiated by electrons from a cathode. A liquid metal, preferably a gallium alloy, film fills a gap between a stationary structure and a back face of the anode, opposite from the track. A cooling fluid, preferably water, is supplied to a cavity behind a wall of the stationary structure. The cooling fluid is thereby in a high thermal conductivity path with the track by way of the wall and liquid metal film.
Houston, U.S. Pat. No. 3,694,685, discloses an X-ray tube having a rotating anode mechanically connected by a high thermal conductivity rotating structure to a gap in a central region of the tube; the gap is filled with a liquid metal film. The gap is between a wall of the rotating structure and a stationary wall of a structure having a cooling fluid, preferably water, flowing through it.
Japanese patent publication 87-194011/28 discloses an X-ray tube having a rotating anode cooled by a vaporizable oil stored in a pool at the bottom of the tube. The oil is pumped as a liquid from the pool so it flows along a back wall of the anode, opposite from the wall containing the X-ray target. The oil is vaporized by heat from the target and then vapor is directed back to the pool. A vacuum pump is connected to the evacuated space to maintain a sufficiently low pressure within the tube.
While the structures of the Houston, German and Japanese references have been suggested, there has been, to our knowledge, no commercialization of the cooling structures disclosed in these patents. For many applications, the structures of these prior art references do not appear to provide adequate cooling of the rotating anode to make investment in use of the liquid metal worthwhile. The corrosive nature of gallium and alloys thereof requires very resistant materials, such as molybdenum, to contact the gallium or gallium alloy. Further, there is no structure disclosed in the German reference or in Houston for adequate confinement of the gallium to the gap between the rotating and stationary parts. In a practical device, gallium and its alloys must be confined because of the highly corrosive properties thereof and because gallium, which in an electrical conductor, may cause electrical shorts in other parts of the tube. In the Japanese reference, the vapor is free to flow over an interior wall of a vacuum envelope including the anode and a cathode.
A number of patents have been issued to Philips relating to an anode disc rotatably journalled on one or more helical-groove bearings. These include the following U.S. Pat. Nos. 4,210,371; 4,375,555; 4,614,445; 4,641,332; 4,644,577; 4,677,651 and 4,856,039 all assigned to US Philips Corporation. It is claimed that X-ray tubes utilizing such bearings have quieter operation and longer life. They have also found that these tubes can operate at higher power levels as more heat is conducted through these bearings than is conducted by ball bearings. These patents do not show or describe ways of providing a high conductivity heat path from the anode track, through a liquid metal film, and then to a high capacity heat exchanger nor do they provide a labyrinth for containing the liquid metal.
It is, therefore, an object of the present invention to provide a new and improved vacuum tube having a rotating anode track bombarded by energetic electrons and cooled with the aid of a liquid metal.
Another object of the invention is to provide a new and improved vacuum tube of the aforementioned type wherein a liquid metal is recirculated through the anode and a heat exchanger to provide considerably greater cooling effects than have been achieved in the prior art.
A further object of the invention is to provide a new and improved vacuum tube of the aforementioned type having improved thermal conducting structures for removing heat from a rotating anode track bombarded by energetic electrons.
Another object of the invention is to provide a new and improved vacuum tube of the aforementioned type wherein a liquid metal film is confined to a gap between a rotating anode region and a stationary wall in the tube.